To obtain the subject high-purity derivative useful for synthesizing medicines, hardly preparing a by-product, in high yield, slight in admixture of position isomer, by reacting a phenol (derivative) as a raw material under a specific condition at two stages.

In production of this compound of formula I (R is H, an alkyl, a cycloalkyl, an aralkyl, an alkoxyl, a halogen, allyl or an aryl; R1 is an OH- protecting group), a phenol or a phenol derivative of formula II is reacted with a base and paraformaldehyde in an organic solvent in the presence of stannous chloride and/or stannic chloride at 60-85°C until the reaction rate reaches 30-80%. Then the reaction is completed at 95-105°C and a prepared salicylaldehyde of formula III is treated with an alkylating agent in water and/or an organic solvent in the presence of a base. A prepared formyl ether of formula IV is oxidized with an oxidizing agent in water and/or an organic solvent and then hydrolyzed with an acid or a base.